Previous work in our Branch and elsewhere has shown that in some individuals psychosis is accompanied by significant elevations in the amount of phenylethylamine (PEA) excreted in the urine. PEA is a metabolite of phenylalanine and is normally produced in both peripheral and central nervous systems. It is rapidly degraded by monoamine oxidase. PEA is known to have many behavioral similarities to amphetamines when injected into rats. In a few individuals with psychosis and elevated urinary PEA, the administration of carbidopa (an L-aromatic amino acid decarboxylase inhibitor) reduces excreted PEA and ameliorates the psychosis. We will identify patients with elevated PEA by screening their urine and then offer them participation in a protocol that will compare carbidopa and neuroleptic in a double- blind fashion. This study is still in the planning phase.